Integrated high throughput cold spray coating manufacturing system

ABSTRACT

In some embodiments, a coating applied to steel reinforcement bar (e.g., steel rebar) that could considerably extend the lifetime of concrete structures by reducing steel rebar corrosion is disclosed. The coating includes a thin, passivating steel (e.g., stainless steel) layer that is applied to the outside of conventional steel rebar. The coating can be applied in-line through metal cold spray manufacturing, which is a high throughput coating technique that can be integrated into existing steel manufacturing plants. Furthermore, a novel, high performance ferritic steel with tailored resistance to corrosion from chlorides is described. The new ferritic steel is distinct from other commercial and experimental steels, and is better suited for coating low-cost steel structures like rebar. Multiple alloying elements including Cr, Al, and Si will each form protective oxides independently, increasing the total amount of protection and extending it over much wider ranges of pH and electrical potential.

PRIORITY

This patent application claims priority from Provisional U.S. PatentApplication No. 63/255,520, filed on Oct. 14, 2021, entitled,“INTEGRATED HIGH THROUGHPUT COLD SPRAY COATING MANUFACTURING SYSTEM,”and naming Samuel McAlpine and Steven Jepeal as inventors, thedisclosure of which is incorporated herein, in its entirety, byreference.

This patent application also claims priority from Provisional U.S.Patent Application No. 63/219,436, filed on Jul. 8, 2021, entitled,“CORROSION-RESISTANT FERRITIC STAINLESS STEEL,” and naming SamuelMcAlpine and Steven Jepeal as inventors, the disclosure of which isincorporated herein, in its entirety, by reference.

This patent application also claims priority from Provisional U.S.Patent Application No. 63/219,434, filed on Jul. 8, 2021, entitled,“STAINLESS-COATED STEEL REINFORCEMENT BAR,” and naming Samuel McAlpineand Steven Jepeal as inventors, the disclosure of which is incorporatedherein, in its entirety, by reference.

BACKGROUND

Steel reinforcement bar (rebar) is used to increase the strength ofconcrete under tension and shear, but the uncoated bar has littleresistance to corrosion. When the steel reinforcement (rebar) corrodes,it expands in volume, and eventually pushes apart and cracks theconcrete around it. Given sufficient corrosion, this leads to majorstructural issues such as spalling of the concrete, where sections ofconcrete break off, and delamination, where wide layers of crackingseparate the reinforcement from the concrete surrounding it. In eithercase, this corrosion causes substantial structural damage to theconcrete, risking catastrophic failure and necessitating major repair orreplacement.

With regard to steel rebar, after fabrication, the bar can corrode inthe presence of atmospheric humidity or rain, leading to the formationof an undesirable iron oxide on the outer surface, which diminishes themarketability of the rebar. After the rebar is impregnated intoconcrete, corrosion can cause substantial structural degradation, oftenthrough pitting corrosion due to the presence of chloride ions. Thislater form of corrosion can severely limit the lifetime of concretestructures using steel rebar.

Several methods exist for improving the corrosion resistance of steelrebar. Many of these options present a high additional cost to the steelrebar and have inherent limitations to efficacy. Epoxy coatings, forexample, are known to delaminate from rebar when in service, andtherefore provide very limited corrosion resistance. The zinc layerproduced during galvanization is prone to attack by the liquid concretemixture during concrete solidification, reducing its efficacy andrequiring thicker applied layers and therefore higher costs. In additionand by way of further example, both galvanization, through the hot dipapplication method, and epoxy coating present additional manufacturingsteps that are not easily integrated into modern, high-throughputmanufacturing methods for steel rebar.

Pure stainless steel rebars and stainless-cladded steel rebars exist,but at a cost that is prohibitively expensive for most applications.

SUMMARY OF VARIOUS EMBODIMENTS

In accordance with an embodiment of the invention, a steel componentincludes a carbon steel reinforcement bar, and an outer layer coatingmetallurgically bonded to the steel component and comprising a ferriticstainless steel. The outer layer coating forms a corrosion resistantcoating on the steel component. A mean thickness of the outer layer isbetween 10 microns and 300 microns.

There may be an interdiffusion region between the carbon steelreinforcement bar and the outer layer coating where a composition of theinterdiffusion region varies continuously from a composition of thecoating to a composition of the carbon steel reinforcement bar. A widthof the interdiffusion region may be between 10 nanometers and 10 microns

The stainless steel coating may include a cold sprayed coating, athermal sprayed coating, a plasma sprayed coating, a laser depositedcoating, a twin wire arc sprayed coating, or an arc welding overlaycoating. The stainless steel coating may have a mean thickness ofbetween 20 microns and 100 microns. The stainless steel coating mayinclude at least one of 316 stainless steel, 2205 stainless steel, or304 stainless steel. The at least one of 316 stainless steel, 2205stainless steel, or 304 stainless steel may be mixed with a metalcarbide or a metal oxide. The metal carbide may include at least one ofchromium carbide, molybdenum carbide, silicon carbide, or manganesecarbide.

In some embodiments, the stainless steel coating includes:

-   -   12-25 weight percent chromium (Cr);    -   2-10 weight percent molybdenum (Mo); and at least one or more        of:    -   0-10 weight percent aluminum (Al);    -   0-5 weight percent silicon (Si);    -   0-5 weight percent nickel (Ni);    -   0-1.0 weight percent manganese (Mn);    -   0.0-0.1 weight percent carbon (C);    -   0.0-0.1 weight percent nitrogen (N); or    -   0.0-0.05 weight percent sulfur (S); and    -   the balance of iron (Fe).

In some embodiments, the stainless steel coating includes:

-   -   16-20 weight percent chromium (Cr);    -   3-6 weight percent molybdenum (Mo); and at least one or more of:    -   0-4 weight percent aluminum (Al);    -   0-2 weight percent silicon (Si);    -   0-0.1 weight percent nickel (Ni);    -   0.1-0.5 weight percent manganese (Mn);    -   0.0-0.1 weight percent carbon (C);    -   0.0-0.1 weight percent nitrogen (N): or    -   0.0-0.05 weight percent sulfur (S); and    -   the balance of iron (Fe).

The stainless steel coating may comprise a face-centered cubic crystalstructure. The stainless steel coating may comprise aferritic/austenitic duplex microstructure. The stainless steel coatingmay be a ferritic stainless steel. The stainless steel coating may havea sufficient ductility that the reinforcement bar may be bent up to 180degrees around an object of a diameter 3.5 times a diameter of thereinforcement bar without visible cracking of the stainless steelcoating. The stainless steel coating may have a sufficient ductilitythat the stainless steel coating has an intrinsic ductility allowing atleast a 5% elongation before failure.

In accordance with another embodiment of the invention, a stainlesssteel coated steel component includes a steel component, and a corrosionresistant ferritic stainless steel coating metallurgically bonded to thesteel component. The stainless steel coating may passivate the steelcomponent against corrosion. The stainless steel coating may be a coldsprayed coating. The stainless steel coating may be a weld overlaycoating. The stainless steel coating may be a twin wire arc spraycoating. The stainless steel coating may be laser cladding. Thestainless steel coating may be thermal spray coating. A mean grain sizeof the stainless steel coating may be between 500 nanometers and 10microns.

In some embodiments, the stainless steel coating includes:

-   -   16-20 weight percent chromium (Cr);    -   3-6 weight percent molybdenum (Mo); and at least one or more of:    -   0-4 weight percent aluminum (Al);    -   0-2 weight percent silicon (Si);    -   0-0.1 weight percent nickel (Ni);    -   0.1-0.5 weight percent manganese (Mn);    -   0.0-0.1 weight percent carbon (C);    -   0.0-0.1 weight percent nitrogen (N): or    -   0.0-0.05 weight percent sulfur (S); and    -   the balance of iron (Fe).

In accordance with another embodiment of the invention, a method offorming a steel component includes providing a steel component having anouter surface, and coating at least a portion of the outer surface withan outer layer including a layer of ferritic stainless steel forming ametallurgical bond to the outer surface. The metallurgically bondedouter layer coating forming a corrosion resistant coating on the steelcomponent.

In some embodiments, the coating may include providing a carrier gas ata high pressure in a first gas flow path to a gas heater to heat thecarrier gas to a high temperature along the first gas flow path. Thecoating may include providing the carrier gas at a high pressure in asecond gas flow path to a particle feeder of stainless steel particlesthat are carried by the carrier gas along the second gas flow path. Thecoating may include mixing the heated carrier gas in the first flow pathwith the carried stainless steel particles in the second flow path at anarray of spray nozzles in fluidic communication with the first andsecond flow paths. The coating may include ejecting a plume of gas andstainless steel particles from the array of nozzles to coat the outersurface of the steel component with a coating of stainless steel as thesteel component is transported through the plume. The ejected stainlesssteel particles may impact the surface of the steel component at a highvelocity and form a metallurgical bond to the surface of the steelcomponent to form an outer coating of ferritic stainless steel. The highvelocity may be a supersonic velocity.

In some embodiments, the array of nozzles circumscribes the steelcomponent to provide coverage of the steel component by the plume. Thehigh pressure may be between about 700 psi and about 800 psi. The hightemperature of the heated gas is between about 900 C and about 1100 C.

In some embodiments, the stainless steel particles may have a meanparticle size of between 5 microns and 25 microns. The stainless steelcoating may have a mean thickness of between 10 microns and 300 microns.Further, the stainless steel coating may have a mean thickness ofbetween 20 microns and 100 microns.

In some embodiments, the component may be a steel rail, a steel beam, asteel girder, a steel rod, a steel bar, or a steel pipe. In someembodiments, the component may be a steel billet.

In some embodiments, the method of forming a steel component may furtherinclude heating the coated steel billet to a temperature between 1000 Cand 1300 C. The method of forming a steel component may further includerolling, sequentially, the coated stainless steel billet into a deformedreinforcement bar. The method of forming a steel component may furtherinclude coating the deformed reinforcement bar with an outer layercomprising a layer of ferritic stainless steel forming a metallurgicalbond to an outer surface of the deformed reinforcement bar. The methodof forming a steel component may further include heat treating thecoated deformed reinforcement bar. The heat treating may include laserheating.

In some embodiments, the steel component includes a steel billet. Thesteel billet may have a rectilinear cross section. The array of spraynozzles may circumscribe the steel billet in a rectilinear configurationto ensure there is an unobstructed line of sight between each region ofthe surface of the billet and at least one of the nozzles in the arrayof nozzles. The stainless steel coating may cover the external surfaceof the steel billet.

In accordance with an embodiment of the invention, a coating depositionsystem for applying a coating of stainless steel to a surface of a steelcomponent includes a gas input for fluidly coupling with a high pressuregas supply configured to provide a gas at a high pressure to one or moreflow paths. The coating deposition system also includes a heated gasflow path in thermal communication with a gas heater, and the heated gasflow path is in fluidic communication with the high pressure gas supply.The gas heater is configured to heat the high pressure flowing gas inthe heated gas flow path. The system also includes a stainless steelparticle feeder flow path in particulate communication with a feederinput for receiving a source of stainless steel particles. The stainlesssteel particle feeder flow path is in fluidic communication with thehigh pressure gas supply. The stainless steel particle feeder isconfigured to supply the stainless steel particles to the high pressureflowing gas in the stainless steel particle feeder flow path.

The system also includes an array of spray nozzles in fluidiccommunication with the heated gas flow path and the stainless steelparticle feeder flow path. The array of spray nozzles is in particulatecommunication with the stainless steel particle feeder flow path. Thearray of spray nozzles is configured to circumscribe the steel componentso that there is an unobstructed line of sight between each region ofthe surface of the steel component and at least one of the nozzles inthe array of nozzles. The array of spray nozzles is also configured toaccelerate the stainless steel particles by a force imparted by a highvelocity of the heated gas exiting each of the nozzles in the array ofnozzles in a plume of heated gas and stainless steel particles. Thearray of spray nozzles is further configured so that the stainless steelparticles impact the surface of the component at the high velocity andmetallurgically bond to the surface of the component to form thestainless steel coating. The stainless steel coating is ferritic,austenitic, or duplex. At least one of the spray nozzles produces astream of stainless steel particles at least partially in a longitudinaldirection and at least partially in a radial direction.

In some embodiments, the array of spray nozzles may be in particulatecommunication with the stainless steel particle feeder flow path. Thearray of spray nozzles may be configured so that for each nozzle in thearray of nozzles the heated gas and particles may enter the nozzle. Theheated gas may be compressed through a converging section of the nozzle.The heated gas may then be expanded through a diverging section of thenozzle. After passing through the converging and diverging sections ofthe nozzle, the heated gas and stainless steel particles may exit thenozzle in the plume and impact the surface of the steel component atsupersonic velocities.

In some embodiments, the system may further include a conveyorconfigured for the transportation of the component through the plume ofhot gas and stainless steel particles. The component may have arectilinear cross section. The array of spray nozzles may be configuredto circumscribe the rectilinear cross section with heads of each of thenozzles in the array of spray nozzles in a rectilinear arrangement.

Further, he component may have a circular, ovular, or deformed-circularcross section. The array of spray nozzles may be configured tocircumscribe the circular, ovular, or deformed-circular cross sectionswith heads of each of the nozzles in the array of spray nozzles in acircular, ovular, or deformed-circular cross section arrangement,respectively.

In some embodiments, the gas may include at least one of nitrogen (N₂),helium (He), air, argon (Ar), xenon (Xe), or forming gas (5% H₂ in N₂).The high pressure may be between about 700 psi and about 800 psi. Thetemperature of the heated gas may be between about 900 C and about 1100C. The high velocity may be supersonic velocity. The stainless steelparticles may have a mean particle size of between 5 microns and 20microns. The stainless steel coating may have a mean thickness ofbetween 0.5 mm and 5 mm. The stainless steel coating may have a meanthickness of between 35 microns and 350 microns. The stainless steelcoating may have a mean thickness of between 25 microns and 300 microns.The stainless steel coating may have a mean thickness of between 10microns and 100 microns. The stainless steel coating may have a BCCferrite matrix.

In some embodiments, the stainless steel particles may include:

-   -   16-20 weight percent chromium (Cr);    -   3-6 weight percent molybdenum (Mo); and at least one or more of:    -   0-4 weight percent aluminum (Al);    -   0-2 weight percent silicon (Si);    -   0-0.1 weight percent nickel (Ni);    -   0.1-0.5 weight percent manganese (Mn);    -   0.0-0.1 weight percent carbon (C);    -   0.0-0.1 weight percent nitrogen (N): or    -   0.0-0.05 weight percent sulfur (S); and    -   the balance of iron (Fe).

In accordance with another embodiment of the invention, a method ofapplying a stainless steel coating to a steel component includesproviding a carrier gas at a high pressure in a first gas flow path to agas heater to heat the carrier gas to a high temperature along the firstgas flow path. The method of applying a stainless steel coating to asteel component includes providing the carrier gas at a high pressure ina second gas flow path to a particle feeder of stainless steel particlesthat are carried by the carrier gas along the second gas flow path. Themethod includes mixing the heated carrier gas in the first flow pathwith the carried stainless steel particles in second flow path at anarray of spray nozzles in fluidic communication with the first andsecond flow paths. The method also includes ejecting a plume of gas andstainless steel particles from the array of nozzles to coat an outersurface of the steel component with a coating of ferritic stainlesssteel as it is transported through the plume. The ejected stainlesssteel particles impact the surface of the steel component at a highvelocity and form a metallurgical bond to the surface of the steelcomponent to form an outer coating comprising ferritic stainless steel.

Each of the nozzles in the array of nozzles may compresses the mixedheated carrier gas and particles through a converging section of eachnozzle. Each of the nozzles in the array of nozzles may expand the mixedheated carrier gas and particles through a diverging section of eachnozzle. Each of the nozzles in the array of nozzles may accelerate themixed heated carrier gas and particles to supersonic velocities.

In some embodiments, the stainless steel particles comprise at least oneof 316 stainless steel, 2205 stainless steel, or 304 stainless steel.

The steel component may be a steel billet. The steel billet may have arectilinear cross section. The array of spray nozzles may circumscribethe steel billet in a rectilinear configuration to ensure there is anunobstructed line of sight between each region of the surface of thebillet and at least one of nozzles in the array of spray nozzles. Thestainless steel coating may cover the entire external surface of thesteel billet, and the thickness of the stainless steel coating may bebetween 150 microns and 500 microns. The stainless steel coating may bebetween 150 microns and 2000 microns.

The method may further include heating the stainless steel coating onthe steel billet at 1200 C for a duration between about 3 hours andabout 9 hours. The method may further include hot rolling the stainlesssteel coating on the steel billet to form a rebar component having thestainless steel coating. In some embodiments, the stainless steelcoating may have a ceramic material alloyed with the stainless steel toimprove the bonding of the stainless steel coating to the steelcomponent. The ceramic material comprises at least one of a metalcarbide or a metal oxide.

In some embodiments, the stainless steel particles may include at leastone of spherical particles fabricated through gas atomization,near-spherical particles fabricated through high pressure wateratomization, or irregular shaped particles fabricated through mechanicalcrushing.

The method may further include heat treating the stainless steel coatingon the steel component. The heat treating the stainless steel coating onthe steel component may include a laser heat treatment.

The heat treating the stainless steel coating on the steel component mayinclude heating the stainless steel coating on the steel component toapproximately 1100 C for 1 hour. The heat treating the stainless steelcoating on the steel component may include quenching the stainless steelcoating on the steel component to room temperature. The heat treatingthe stainless steel coating on the steel component may include temperingthe stainless steel coating on the steel component at approximately 600C for 1 hour.

In accordance with an embodiment of the invention, a corrosion resistantstainless steel alloy composition having a BCC ferrite matrix includes:

-   -   12-25 weight percent chromium (Cr);    -   2-10 weight percent molybdenum (Mo); and at least one or more        of:    -   0-10 weight percent aluminum (Al);    -   0-5 weight percent silicon (Si);    -   0-5 weight percent nickel (Ni);    -   0-1.0 weight percent manganese (Mn);    -   0.0-0.1 weight percent carbon (C);    -   0.0-0.1 weight percent nitrogen (N); and    -   0.0-0.05 weight percent sulfur (S); and    -   the balance of iron (Fe).

In some embodiments, the corrosion resistant stainless steel alloycomposition having a BCC ferrite matrix may include:

-   -   16-20 weight percent chromium (Cr);    -   3-6 weight percent molybdenum (Mo); and at least one or more of:    -   0-4 weight percent aluminum (Al);    -   0-2 weight percent silicon (Si);    -   0-0.1 weight percent nickel (Ni);    -   0.1-0.5 weight percent manganese (Mn);    -   0.0-0.1 weight percent carbon (C);    -   0.0-0.1 weight percent nitrogen (N):    -   0.0-0.05 weight percent sulfur (S); and    -   the balance of iron (Fe).

In some embodiments, the corrosion resistant stainless steel alloycomposition having a BCC ferrite matrix may include:

-   -   18 weight percent chromium (Cr);    -   6 weight percent molybdenum (Mo);    -   4 weight percent aluminum (Al);    -   2 weight percent silicon (Si); and    -   the balance of iron (Fe).

In some embodiments, the corrosion resistant stainless steel alloycomposition having a BCC ferrite matrix may include:

-   -   18 weight percent chromium (Cr);    -   3 weight percent molybdenum (Mo);    -   4 weight percent aluminum (Al);    -   2 weight percent silicon (Si); and    -   the balance of iron (Fe).

In some embodiments, the corrosion resistant stainless steel alloycomposition having a BCC ferrite matrix may include:

-   -   18 weight percent chromium (Cr);    -   8 weight percent molybdenum (Mo);    -   5 weight percent aluminum (Al); and    -   the balance of iron (Fe).

In some embodiments, the corrosion resistant stainless steel alloycomposition having a BCC ferrite matrix may include:

-   -   18 weight percent chromium (Cr);    -   8 weight percent molybdenum (Mo);    -   2 weight percent silicon (Si); and    -   the balance of iron (Fe).

In some embodiments, the corrosion resistant stainless steel alloycomposition having a BCC ferrite matrix may include:

-   -   18 weight percent chromium (Cr);    -   4 weight percent molybdenum (Mo); and    -   the balance of iron (Fe).

In accordance with another embodiment of the invention, a method ofmaking a corrosion resistant ferritic BCC stainless steel alloy includesproviding a metal mixture that includes:

-   -   12-25 weight percent chromium (Cr);    -   2-10 weight percent molybdenum (Mo); and at least one or more        of:    -   0-10 weight percent aluminum (Al);    -   0-5 weight percent silicon (Si);    -   0-5 weight percent nickel (Ni);    -   0-1.0 weight percent manganese (Mn);    -   0.0-0.1 weight percent carbon (C);    -   0.0-0.1 weight percent nitrogen (N); or    -   0.0-0.05 weight percent sulfur (S); and    -   the balance of iron (Fe).

The method of making a corrosion resistant ferritic BCC stainless steelalloy also includes providing a furnace for melting the metal mixture,heating the metal mixture in the furnace to form a liquid metal mixturemelt, and cooling the liquid metal mixture melt to form a solid metalmixture. The solid metal mixture comprises a corrosion resistantferritic stainless steel alloy. The furnace may be a vacuum inductionmelting furnace. The furnace may be a vacuum arc melting furnace.Cooling the liquid metal mixture melt may include quenching the liquidmetal mixture melt.

The method of making a corrosion resistant ferritic BCC stainless steelalloy may further include atomizing the corrosion resistant ferriticstainless steel alloy to produce corrosion resistant stainless steelalloy particles. The method may include providing the corrosionresistant stainless steel alloy particles to a cold spray system. Themethod may include coating a steel component with the corrosionresistant stainless steel alloy particles ejected from the cold spraysystem. The ejected corrosion resistant stainless steel alloy particlesmay metallurgically bond to an outer surface of a steel component toform a corrosion resistant stainless steel coating having a BCC ferritematrix on the steel component.

The method of making a corrosion resistant ferritic BCC stainless steelalloy may further include heat treating the corrosion resistantstainless steel coating having a BCC ferrite matrix on the steelcomponent. The heat treating the corrosion resistant stainless steelcoating having a BCC ferrite matrix may include a laser heat treatment.

The heat treating the corrosion resistant stainless steel coating havinga BCC ferrite matrix may include heating the corrosion resistant coatingon the component to a temperature between approximately 1000 C andapproximately 1300 C for a duration between 1 hour and 24 hours. Theheat treating may include quenching the corrosion resistant coating onthe steel component. The heat treating may include tempering thecorrosion resistant coating on the steel component at a temperaturebetween 400 C and 700 C for a duration between 10 minutes and 4 hours.

The atomizing the corrosion resistant stainless steel alloy may includeat least one of gas atomizing the corrosion resistant ferritic stainlesssteel alloy to produce spherical particles of the corrosion resistantstainless steel alloy, atomizing the corrosion resistant ferriticstainless steel alloy to produce near-spherical particles of thecorrosion resistant stainless steel alloy, or mechanically crushing thecorrosion resistant ferritic stainless steel alloy to produce irregularshaped particles of the corrosion resistant stainless steel alloy.

The corrosion resistant stainless steel alloy particles may have a meanparticle size of between 5 microns and 20 microns. The stainless steelcoating may have a mean thickness of between 10 microns and 500 microns.

The component may be a steel billet. The component may be a steel rebar,a steel beam, a steel rail track, or a steel pipe.

In accordance with another embodiment of the invention, a method ofmaking a ferritic stainless steel alloy includes providing a metalmixture comprising:

-   -   12-25 weight percent chromium (Cr);    -   2-10 weight percent molybdenum (Mo); and at least one or more        of:    -   0-10 weight percent aluminum (Al);    -   0-5 weight percent silicon (Si);    -   0-5 weight percent nickel (Ni);    -   0-1.0 weight percent manganese (Mn);    -   0.0-0.1 weight percent carbon (C);    -   0.0-0.1 weight percent nitrogen (N); or    -   0.0-0.05 weight percent sulfur (S); and    -   the balance of iron (Fe).

The method of making a ferritic stainless steel alloy also includesproviding a furnace for melting the metal mixture. The metal mixture isheated in the furnace to a temperature between about 1600 C and about2000 C to form a liquid metal mixture melt. The liquid metal mixturemelt is cooled to an intermediate temperature between about 1000 C andabout 1300 C over a first duration of time to initiate a solidificationprocess. The liquid metal mixture melt is held at the intermediatetemperature between about 1000 C and about 1300 C for a second durationof time. The liquid metal mixture melt is quenched to a temperaturebetween about 400 C and about 600 C over a third duration of time. Thefirst duration of time is between may be between 5 minutes and 100minutes. The second duration of time may be between 0.5 seconds and 10.0seconds

The quenching limits the formation of carbide precipitates. The metalmixture is tempered at a temperature between about 450 C and about 600 Cfor a duration of time of between about 10 minutes and about 60 minutes.The metal mixture is cooled in the absence of active heating. The metalmixture includes ferritic stainless steel alloy. The ferritic stainlesssteel alloy may be corrosion resistant. The ferritic stainless steelalloy may have a body centered cubic crystal structure.

The furnace may include a cold sprayer. The method may further includeatomizing the ferritic stainless steel alloy. The method may furtherinclude depositing the atomized ferritic stainless steel alloy as acoating on a steel component. The ferritic stainless steel alloy coatingmay be metallurgically bonded to the surface of the steel component. Theferritic stainless steel alloy coating on the steel component may resistcorrosion of the steel component. The ferritic stainless steel alloy maybe a bulk material.

BRIEF DESCRIPTION OF THE DRAWINGS

Those skilled in the art should more fully appreciate advantages ofvarious embodiments of the invention from the following “Description ofIllustrative Embodiments,” discussed with reference to the drawingssummarized immediately below.

FIG. 1 schematically illustrates various embodiments of the presentdisclosure of steel rebar with a stainless steel coating in accordancewith illustrative embodiments,

FIG. 2 schematically illustrates various embodiments of the presentdisclosure of an interfacial region between a protective coating andconcrete in accordance with illustrative embodiments,

FIG. 3 schematically illustrates various embodiments of the presentdisclosure of an interfacial region between a protective coating andconcrete in accordance with illustrative embodiments,

FIG. 4 schematically illustrates various embodiments of the presentdisclosure of an interfacial region between a protective coating andconcrete in accordance with illustrative embodiments,

FIG. 5 schematically illustrates various embodiments of the presentdisclosure of an interfacial region between a protective coating andconcrete in accordance with illustrative embodiments,

FIG. 6 schematically illustrates a corrosion holiday and pittingcorrosion,

FIG. 7 schematically illustrates various embodiments of the presentdisclosure of a coating interface in the case of a cold sprayapplication in accordance with illustrative embodiments,

FIG. 8 schematically illustrates a poorly bonded and porous coating,

FIG. 9 schematically illustrates various embodiments of the presentdisclosure of a cold spray deposition system in accordance withillustrative embodiments,

FIG. 10A schematically illustrates various embodiments of the presentdisclosure of a cold spray deposition system in accordance withillustrative embodiments,

FIG. 10B schematically illustrates various embodiments of the presentdisclosure of a cold spray deposition system in accordance withillustrative embodiments,

FIG. 11 schematically illustrates various embodiments of the presentdisclosure of a nozzle design in accordance with illustrativeembodiments,

FIG. 12A schematically illustrates various embodiments of the presentdisclosure of nozzle arrays in accordance with illustrative embodiments,

FIG. 12B schematically illustrates various embodiments of the presentdisclosure of nozzle arrays in accordance with illustrative embodiments,

FIG. 13 (left) schematically illustrates various embodiments of thepresent disclosure of nozzle arrays in accordance with illustrativeembodiments,

FIG. 13 (right) schematically illustrates various embodiments of thepresent disclosure of nozzle arrays in accordance with illustrativeembodiments,

FIG. 14 (left) schematically illustrates various embodiments of thepresent disclosure of nozzle arrays in accordance with illustrativeembodiments,

FIG. 14 (right) schematically illustrates various embodiments of thepresent disclosure of nozzle arrays in accordance with illustrativeembodiments,

FIG. 15 schematically illustrates various embodiments of the presentdisclosure of nozzle arrays in accordance with illustrative embodiments,

FIG. 16 schematically illustrates various embodiments of the presentdisclosure of nozzle arrays in accordance with illustrative embodiments,

FIG. 17A schematically illustrates various embodiments of the presentdisclosure of an integration of spray systems into a manufacturing linein accordance with illustrative embodiments,

FIG. 17B schematically illustrates various embodiments of the presentdisclosure of an integration of spray systems into a manufacturing linein accordance with illustrative embodiments,

FIG. 18 schematically illustrates various embodiments of the presentdisclosure of a corrosion resistant stainless steel alloy in accordancewith illustrative embodiments,

FIG. 19 shows an Ellingham diagram presenting the relative driving forcefor the oxide formation among the constituent elements in the presentinvention,

FIG. 20 shows equilibrium phase compositions for ranges of Cr and Mocontents in stainless steel alloys of iron, chromium, and molybdenum,

FIG. 21 shows an equilibrium phase fraction plot for an exampleFe—Cr—Mo—Al—Si composition demonstrates the secondary phases that shouldbe limited or avoided completely,

FIG. 22 illustrates time-temperature-transformation curve for an exampleFe—Cr—Mo—Al—Si composition,

FIG. 23 shows a phase composition map for Fe-18Cr-3Mo-4Al-2Si shows thepresence of deleterious secondary phases at low temperatures,

FIG. 24 shows a time-temperature-transformation diagram forFe-18Cr-3Mo-4Al-2Si,

FIG. 25 shows an equilibrium phase fraction plot for an exampleFe—Cr—Mo—Al composition demonstrates the secondary phases that should belimited or avoided completely,

FIG. 26 shows a time-temperature-transformation diagram forFe-18Cr-8Mo-5Al,

FIG. 27 illustrates an equilibrium phase fraction plot for anFe—Cr—Mo—Si alloy demonstrates the secondary phases that should belimited or avoided completely,

FIG. 28 shows a time-temperature-transformation diagram forFe-18Cr-8Mo-2Si,

FIG. 29 illustrates an equilibrium phase fraction plot for an Fe—Cr—Moalloy demonstrates the secondary phases that should be limited oravoided completely,

FIG. 30 shows a time-temperature-transformation diagram for Fe-18Cr-4Mo.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

In illustrative embodiments, a coating is applied to steel reinforcementbar (e.g., steel rebar) that could considerably extend the lifetime ofconcrete structures by reducing steel rebar corrosion. The coatingincludes a thin, passivating steel (e.g., stainless steel) layer that isapplied to the outside of conventional steel rebar. The coating can beapplied in-line through metal cold spray manufacturing, which is a highthroughput coating technique that can be integrated into existing steelmanufacturing plants. The technology solves the challenge of rebarcorrosion in the presence of water and chlorides (salt), which limitsthe service life of many concrete structures including bridges,roadways, and marine structures. The coated rebar has a longer lifetimeand lower cost than conventional coatings like epoxy coating. Thein-line coating application method that can be directly integrateddirectly into steel manufacturing plants.

In illustrative embodiments, a high performance ferritic steel withtailored resistance to corrosion from chlorides is described. The newferritic steel is distinct from other commercial and experimentalsteels, and is better suited for coating low-cost steel structures likerebar. This new steel provides complete protection against corrosion inthe presence of chlorides. The new ferritic steel coating is separatefrom conventional steels because it is formulated to perform as a thincoating designed to produce multiple protective oxide layers to givemore extensive corrosion protection than existing stainless steels.Multiple alloying elements including Cr, Al, and Si will each formprotective oxides independently, increasing the total amount ofprotection and extending it over much wider ranges of pH and electricalpotential. High concentrations of Mo are included to reverse any pittingcorrosion caused by chlorides by re-passivating newly formed pits. Theresulting steel is more protective against corrosion, especially in highconcentrations of chloride ions, than conventional stainless steels orgalvanized steels. The novel, high performance ferritic steel can alsobe applied in-line through metal cold spray manufacturing to resistcorrosion to the rebar.

It should be noted, however, that although steel reinforcement bar isdiscussed, various embodiments apply to other types of steel products,such as I-beams, rails, pipes, billets, tubes, and the like.Accordingly, discussion of steel reinforcement bar is for illustrativepurposes and not intended to apply to all embodiments.

Corrosion Resistance

Corrosion of rebar occurs through at least two pathways. One pathway isoxidation of the surface of the steel that occurs through exposure towater. Another pathway is a pitting pathway where chloride ion attacksthe surface of the steel in pits formed in the surface of the rebar. Thecorrosion caused by pitting is a result of chloride ion (e.g. Cl⁻)corrosion is due to exposure to salts such as sodium chloride (e.g.,NaCl). The chloride ion can get access to the surface iron on the steel

The oxidation pathway can be blocked by the formation of oxide barrierson the surface of the steel that forms a passivating layer (e.g.,passivation coating). Passivating layers of certain metal oxides, suchas chromium oxide, Cr₂O₃ (e.g. chromia), silicon dioxide, SiO₂ (e.g.silica), and aluminum oxide Al₂O₃ (e.g. alumina) on the surface of steelcan protect the steel against oxidative corrosion. The pitting corrosionspecifically can be blocked by certain metal oxides, such as molybdenumoxide, MoO₃ that are resistant to chloride ion attack (e.g. corrosion).

For passivation layers to be effective at preventing corrosion, theymust be strongly bonded to the steel surface, they must fully cover thesurface, and they must be sufficiently thick to provide a robustcoating. To produce a passivation layer, a metallic coating can beapplied that will naturally produce the desired oxide layer. Strongbonding between the metallic coating and the underlying component can beachieved by a metallurgical bond (e.g. metallic bond). Certaintechniques, such as cold spray and weld overlay are capable of applyinga metal coating to the steel that is metallurgically bonded (e.g.metallically bonded). Furthermore, these techniques are also capable ofdepositing a coating over an entire surface of a steel component, suchas rebar or a billet. In addition, these techniques (cold spray and weldoverlay) are capable of applying a metal coating to the steel that issufficiently thick to provide a robust corrosion resistance, oftenbetween 50 and 300 micrometers.

In embodiments, a metal coating, such as a stainless steel, is appliedto a steel component, and the metal coating forms a native oxide coatingthat serves as the passivating layer. A native oxide coating is a layerof one or more metal oxides that is formed on the surface of the metalspontaneously in water or air due to the exothermic driving force of theformation of the metal oxide. The oxygen in the water or air oxidizesthe surface atoms of the metal to form a thermodynamically stable oxidecoating. This surface oxide forms whenever a bare metal atom is exposedby either being freshly deposited, or by the protected oxide layer beingscratched, or otherwise penetrated.

An example of this process is the deposition of a chromium containingstainless steel on a steel component. In this example, the depositedstainless steel coating includes chromium metal, of which some is on thesurface of the coating. Oxygen present in the water or air spontaneouslyforms a surface oxide of chromium (e.g., chromium oxide, Cr₂O₃), and itis the surface layer of the chromium oxide layer that is formedspontaneously in reaction with the oxygen in the water or air thatprovides the protection from oxidation and pitting corrosion.

This process of forming a surface oxide from atoms on the surface of thestainless steel occurs spontaneously. Furthermore, the thermodynamicstrength of the bonding between the metal and the oxygen can result in arearrangement of the surface atoms to form layers of a certain metaloxide. In the example above describing the formation of chromium oxide,the thermodynamic driving force of the formation of the Cr₂O₃ may causethe metal atoms at the surface of the stainless steel coating torearrange (e.g., diffuse) to allow the formation of the chromia layer.That is, the meal atoms close to the surface of the alloy may diffusefrom a location below the surface of the alloy to the surface under thethermodynamic potential of forming a native metal oxide on the surfaceof the alloy.

The thermodynamic driving force for the formation of an oxide of eachatom that makes up a stainless steel coating can cause the metal oxidelayers to separate and stack upon each other in a predictable order toform a multilayer structure spontaneously. The order of the stacking ofthe metal oxide layers in the multilayer is determined by free energy offormation (ΔG) for each of the metal oxides. The metal oxide with thelargest ΔG (in absolute value) will tend to form closest to the steelcomponent, with the metal oxide having the smallest absolute ΔG on thesurface of the coating. That is, the metal atoms diffuse up frompositions in the alloy structure up through the metal alloy to thesurface to form each metal oxide layer, and the position of a particularmetal oxide in the multilayer is determined by the ΔG of each metaloxide.

In some embodiments, the incorporation of certain metals as primaryalloying metals, such as chromium and molybdenum, into stainless steelcoatings with a base metal of iron may also provide structuraladvantages, as well as corrosion resistance. Herein, a variety ofstainless steel coatings are disclosed with different elements asprimary (e.g., chromium (Cr), molybdenum (Mo), nickel (Ni), and/ormanganese (Mn), and the like) and secondary and tertiary alloyingelements (e.g., aluminum (Al), silicon (Si), carbon (C), nitrogen (N),and/or sulfur (S), and the like) and with different compositions of thealloys. In some embodiments, the primary alloying elements may beprimary oxide formers, while secondary alloying elements may besecondary oxide formers.

Bi-Metal Steel Composite

In order to impart robust corrosion resistance to a steel component(e.g., steel reinforcement bar (rebar)) at a low cost, illustrativeembodiments utilize a thin outer coating of stainless steel ontoconventional carbon steel rebar. This thin coating can passivate thesteel rebar in the same manner as a pure stainless steel rebar,providing resistance to corrosion before and after consumption inconcrete structures. This results in concrete structures that are moreresilient to corrosion, and therefore have extended operationallifetimes.

FIG. 1 schematically illustrates steel rebar with a stainless steelcoating 100. FIG. 1 (left) schematically illustrates a cross sectionalview of a corrosion resistant stainless steel coated bar, and FIG. 1(right) schematically illustrates a plan view of a portion of a surfacecorrosion resistant stainless steel coated bar. The steel rebar 110 hasribbing 120, and the rebar is illustrated with a stainless steel coating130. The stainless steel coating 130 completely covers the steel rebar110 including the ribbing 120. Complete coverage of the rebar 110 withits ribs 120 by the stainless steel coating 130 is necessary forcomprehensive corrosion resistance.

In some embodiments, the use of a molybdenum-containing coating (e.g.outer layer) imparts resistance to chloride attack. This providesresistance to salt-containing environments, such as marine applicationsand locations that use de-icing salts. In some embodiments, the use ofsecondary oxide formers including aluminum and silicon bolsters thecorrosion resistance across a wider range of environments.

FIG. 2 schematically shows an embodiment of an interfacial regionbetween a protective coating and concrete in accordance withillustrative embodiments. FIG. 2 (left) schematically illustrates theinterface 210 between a coating 220 on a coated steel rebar and concrete230. The interfacial region 210 is abrupt, indicating that formation ofa passivation layer has not started. FIG. 2 (right) schematicallyillustrates the interface 240 between a coating 220 on a coated steelrebar 250 and concrete 230. A protective native oxide layer 260 (e.g.,passivation layer) has grown on the surface of the coated rebar 220 as aresult of the oxidizing components in the concrete. The protective oxidelayer 260 provides resistance to corrosion of the underlying rebar 250.The outer oxide layer 260 contains passivating compounds (e.g., forexample, Cr₂O₃ and MoO₃) that protect against corrosion, and preventpotentially corrosive species from coming into contact with the rebar.These compounds form native protective oxide layers outside of thecoating at the interface between the coating 220 and the concrete 230,thereby preventing corrosion of the underlying carbon steel rebar.

FIG. 3 schematically shows an embodiment of an interfacial regionbetween a protective coating and concrete in accordance withillustrative embodiments. FIG. 3 schematically illustrates 300 aprotective oxide layer 360 of Cr₂O₃ having formed on a Fe—Cr alloy. Thenative protective oxide layer 360 of Cr₂O₃ forms on the surface of theFe—Cr alloy because of the thermodynamic force to form an oxide compoundwhen oxidizing elements (e.g., oxygen) are in contact with metallicelements (e.g., chromium, molybdenum, and the like). The protectiveoxide layer 360 of Cr₂O₃ is a passivating layer that provides a barrierthat limits the diffusion of oxygen from the concrete 330 to the metalsurface of a steel component (e.g., rebar, steel billet, and the like).The passivating Cr₂O₃ layer thus limits the rate of corrosion. FIG. 3schematically illustrates how chromium in the outer coating 320 forms achromia (Cr₂O₃) 360 outer layer, preventing and/or substantiallymitigating the corrosion of iron within the steel component core.

In some embodiments, secondary oxide formers, such as silicon andaluminum, can be added to stainless steel alloys to bolster the chromiumoxide and/or molybdenum oxide layers with additional protective oxides.The function of these secondary oxide formers is schematicallyillustrated in FIG. 4 .

FIG. 4 schematically shows an embodiment of an interfacial regionbetween a protective coating and concrete in accordance withillustrative embodiments. FIG. 4 schematically illustrates 400 aprotective oxide 460 of Cr₂O₃/Al₂O₃/SiO₂ having formed on a Fe—Cr—Si—Alalloy. The protective oxide 460 of Cr₂O₃/Al₂O₃/SiO₂ forms a passivatingmultilayer (the multilayer structure is not shown in FIG. 4 ) thatprovides a barrier that limits the diffusion of oxygen from the concrete430 to the iron core metal surface. The passivating Cr₂O₃/Al₂O₃/SiO₂multilayer limits the rate of corrosion. FIG. 4 schematicallyillustrates how chromium, silicon, and aluminum in the outer coating 420forms a Cr₂O₃/Al₂O₃/SiO₂ 360 multi-oxide layer barrier, preventingand/or substantially mitigating the corrosion of iron within the coatingor rebar core.

In addition to oxygen in the air and concrete, chloride ions (e.g., Cl⁻)present in many salts leads to corrosion of steel components. Thechloride ions can form metal chlorides that corrodes steel components,as well as form metal oxy-chlorides with oxygen that is very effectiveat corroding and weakening structural steel, such as rebar.

Molybdenum oxide (e.g., MoO₃) is an effective barrier oxide to preventchloride ions from corroding steel. Molybdenum metal forms a nativeoxide barrier layer, molybdenum oxide (e.g., MoO₃), on stainless steelalloys that contain molybdenum. In some embodiments, molybdenum in thecoating can diffuse to pits in the coating and form molybdenum oxide.This MoO₃ re-passivates pits that form, preventing the growth of pittingtype corrosion. This is particularly relevant in chloride environments,where pitting type corrosion can breach the protective oxide layer andcompromise the underlying steel component (e.g., rebar). Thisre-passivation of pitting by molybdenum is depicted in FIG. 5-1 .

FIG. 5 schematically shows an embodiment of an interfacial regionbetween a protective coating and concrete in accordance withillustrative embodiments. FIG. 5 schematically illustrates 500 aprotective oxide layer 560 of Cr₂O₃ having formed on a Fe—Cr—Mo alloy.The native protective oxide layer 560 of Cr₂O₃ forms on the surface ofthe Fe—Cr—Mo alloy because of the thermodynamic force to form thechromium oxide compound when oxidizing elements (e.g., oxygen) are incontact with metallic elements (e.g., chromium). The protective oxidelayer 560 of Cr₂O₃ is a passivating layer that provides a barrier thatlimits the diffusion of oxygen from the concrete 530 to the metalsurface of a steel component (e.g., rebar, steel billet, and the like).The passivating Cr₂O₃ layer thus limits the rate of corrosion at leastfrom oxygen.

FIG. 5 also schematically illustrates how oxide formation on the surfaceof the Fe—Cr—Mo alloy can reduce corrosion from pit/crevice formation inthe alloy coating. A crevice/pit 570 may be formed through the oxidepassivation coating Cr₂O₃ 560 and into the Fe—Cr—Mo alloy coating 520.As schematically illustrated, molybdenum oxide (e.g., MoO₃) in stainlesssteel alloy may form a re-passivation layer 580 that prevents and/orsubstantially reduces corrosion of the underlying steel component fromthe Cl⁻ ion. That is, the presence of Mo in the stainless steel alloyprovides a metal (Mo) that may diffuse to the surface of a pit/creviceto form an effective oxide layer of MoO₃ that may increase corrosionresistance in the presence of chloride ions.

Thus, while the formation of pits and/or crevices in a steel componentmay greatly increase the corrosion of the steel component, theapplication of a stainless steel barrier on the steel component, asdisclosed herein in multiple embodiments, may significantly reduce, oreven prevent corrosion of the steel component in the presence ofpits/crevices. With successful metallurgical bonding, the stainlesssteel coating will remain attached to the bar through deformation of thebar, such as bending. A complete coating is expected to performcomparably to (or as well as) a bar made entirely of stainless steel,but with a substantially lower fraction of stainless steel (e.g., nomore than 5%). With successful metallurgical bonding, the coating isexpected to perform far better than modern epoxy-coated reinforcementbars due to the tough, robust nature and inherent corrosion-resistanceof the stainless steel coating.

General Method of Fabrication

At least two general approaches to fabricate the coated steel componentsdisclosed herein in illustrative embodiments include a spray depositiontechnique and a weld overlay deposition technique. In the firstapproach, a spray technique such as cold spray is used on a finished ornearly finished steel component, such as rebar. In the second approach,a weld overlay deposition technique is used to deposit acorrosion-resistant coating in a thickness that is larger than thethickness of the finished product. The following discussion of these twotechniques are in no way limiting, and those skilled in the art may useother approaches to form the stainless steel coating disclosed herein.These two approaches are discussed for illustrative purposes only.

The first approach, using cold spray to deposit the coating onto thefinished bar, is an excellent choice when the throughput required formanufacturing the component is high.

The second approach of using weld overlay, or some other coatingdeposition method/technique, followed by sequential rolling is a betterchoice when the quality of the coating is critical, but a lowerthroughput process is acceptable. Generally, the weld overlay processtakes a much longer time to deposit the coating than cold spray.

When employing the first approach of cold spray deposition, thedeposited particles are propelled at a high enough speed to achieve theadiabatic shear instability upon impact, which allows for plasticdeformation to occur. This causes cold welding (e.g., metallurgicalbonding) between the metallic coating and the underlying steelcomponent.

When employing the second approach of weld overlay, special attention ispaid to the heat-affected zone and potential carbide formation in thecorrosion resistant coating. Carbide formation can potentially lead tocracking of the coating during the subsequent rolling process. Shouldcarbide formation occur, heat treatment of the overlaid billet (e.g., at1200 Celsius for 1 hour) can be employed. The heat treatment can befollowed by several sequential rolling steps which reduce the total barthickness as well as the coating thickness.

Coating Requirements

The corrosion-resistant metallic coating must completely cover thesurface of the bar. If the coating is not complete, small openings or“holidays” can lead to a pitting corrosion attack. A pit (e.g., crevice)can lead to significant corrosion, particularly in the presence ofchlorides (e.g., chloride ions, Cl⁻). An example of this pitting processresulting from a coating holiday is shown in FIG. 6 .

FIG. 6 schematically shows 600 a coating holiday, illustrating apotential for pitting corrosion from coating defects. In the event thata corrosion resistant stainless steel coating 620 on a steel component650 has a coating holiday 690, it is possible for a pitting corrosion695 to be initiated. Attacks to the structural integrity of the steelcomponent by chloride ions and/or oxygen can proceed to create pittingcorrosion 695. Such coating holidays may occur when the coverage of thecoating is incomplete, and/or when the coating is poorly bonded or isporous.

FIG. 7 schematically illustrates various embodiments of the presentdisclosure of a coating interface in the case of a cold sprayapplication in accordance with illustrative embodiments. A schematiccross-sectional view 700 of the coating interface 715 for a cold sprayapplication is shown. In cold spray deposition, a full contact surfaceinterface 715 develops between the particles 705 and the surface of thesubstrate (e.g., steel component) 750. The coated particles 705 areflattened, and the flattening is indicative of plastic deformation andthe cold welding of the impacted particle 705 to the substrate 750,which results in a well bonded coating 720.

In some embodiments, a mean particle size of the particles 705 may bebetween 5 microns and 20 microns. The corrosion resistant stainlesssteel coating 720 may have a mean thickness of between 10 microns and100 microns. The corrosion resistant stainless steel coating 720 isfully adhered to the bar 750 with metallurgical bonding at the interfacebetween the coating 720 and the underlying steel bar (e.g., component)750.

FIG. 8 schematically illustrates 800 a poorly bonded and porous coating820. Poorly bonded, porous coatings 820 are undesirable and may lead todetachment of the coating 820 or penetration of the coating 820 byliquid in the corrosive environment. In unsuccessful bonding, thecoating will not cover the surface 850 of the steel component, and/orwill not adhere to the underlying substrate at the interface 815. Theremay be pores 825 which allow corrosive liquids and/or gases to penetratethe poorly bonded coating 820 and corrode the underlying steel component850.

Coating Materials

In some embodiments, a range of coating metals can provide corrosionresistance. For example, ferritic stainless steels including grade 430steel, iron-chromium-aluminum alloys, iron-chromium-molybdenum-aluminumalloys, iron-chromium-molybdenum-aluminum-silicon alloys, and the like,provide the effective corrosion resistance to steel components, asdescribed herein. Furthermore, a ferrous alloy (e.g., with a minimum 14wt % Cr) significantly improves the corrosion resistance of thereinforcement bar. In high-chloride environments, such as saltwaterenvironments or roads, a chloride resistant coating alloy with a minimum2 wt % Mo could be used to withstand chloride attack.

Further, new ferritic stainless steel alloys with a body centered cubic(e.g., BCC) structure are disclosed below. Other embodiments utilizeMartensitic steels such as grade 4130 steel. Still, other embodimentsutilize austenitic stainless steels such as grade 304 and grade 316steel. Still, other embodiments utilize non-ferrous metals and alloys,such as aluminum, titanium, and chromium based metals. The above coatingmetals and those described in the examples below are for illustrativepurposes only. Those skilled in the art may use other coating materialsnot listed here.

Cold Spraying System

Illustrative embodiments relate to a process for high throughput metalcoating of metal components through continuous cold spray additivemanufacturing. Illustrative embodiments also relate to componentsrelated to this process.

Many low cost steel structures suffer from reduced operational lifetimesdue to the effects of corrosion on structural integrity. It can often bebeneficial to apply a corrosion resistant outer layer to extend theperformance lifetime. Existing methods including epoxy coating,painting, and galvanization provide limited resistance and large costsdue to the inability to easily integrate into modern, high throughputmethods of steel fabrication.

Cold spray additive manufacturing is a relatively high throughput methodof applying a metal coating to the surface of a component. In coldspray, metal particles are sprayed out of a nozzle of the spray coateronto a substrate at high speeds and temperatures well below the metalmelting point. These particles are accelerated to high speeds by asupersonic carrier gas before impacting the substrate. The coatingparticles then bond to the substrate upon impact, when their kineticenergy drives severe plastic deformation and cold welding. The coatingparticles form a metallurgical bond (e.g., metallic bond) with thesubstrate.

FIG. 9 schematically illustrates various embodiments of the presentdisclosure of a cold spray deposition system 900 in accordance withillustrative embodiments. In some embodiments, a cold spray depositionsystem 900 includes a spray gun 910. The spray gun 910 includes a gasinput 912 that allows connection of a gas source to the spray gun 910.The gas may include at least one of nitrogen (N₂), helium (He), air,argon (Ar), xenon (Xe), or forming gas (5% H₂ in N₂). The gas may be ata high pressure between about 700 psi and about 800 psi. The gas may beheated by an energy source 914 to temperatures between about 900 C andabout 1100 C.

The spray gun 910 includes a feeder input 918 for receiving a source ofstainless steel particles from a particle feeder. The particle feedermay be a hopper for providing the stainless steel particles to the spraygun 910. The stainless steel particles may have a mean particle size ofbetween 5 microns and 20 microns. The feeder feeds the stainless steelparticles in a flow line that has the high-pressure gas that is acarrier gas for the particles.

The spray gun 910 includes an array of spray nozzles (not shown) that isconfigured to circumscribe a steel component so that there is anunobstructed line of sight between each region of the surface of thesteel component and at least one of the nozzles in the array of nozzles.

The high-pressure, heated gas that is carrying the particles is fed intothe array of nozzles. The array of spray nozzles is configured toaccelerate the stainless steel particles by a force imparted by a highvelocity of the heated gas exiting each of the nozzles in a plume ofheated gas and stainless steel particles 920. The stainless steelparticles impact the surface of the component 930 at the high velocityand metallurgically bond to the surface of the component 930 to form astainless steel coating 940. In some embodiments the high velocity is asupersonic velocity.

The component 930 travels through the spray plume 920 with a componentmotion 950. The component motion 950 may be provided by a conveyor orsome other mechanism to move the metal component 930 through the sprayplume 920.

An expanded view 960 of an embodiment of the spray deposition system 900shows particles 925 moving toward the component 930, as indicated by thearrows. The corrosion resistant coating 940 is shown being deposited onthe component 930 as a buildup of particles 925 on the component. Theparticles 925 have a flattened appearance because they arrive at thecomponent 930 at supersonic velocities. The flattened appearance isindicative of plastic deformation and the cold welding of the impactedparticle 925 to the component (e.g., substrate) 930, which results in awell bonded coating 940.

Sprayer Components

FIGS. 10A and 10B schematically illustrate various embodiments of thepresent disclosure of a cold spray deposition system in accordance withillustrative embodiments. As shown in FIGS. 10A and 10B, a spray system,1000 and 1010, respectively, includes multiple components including agas supply 1012, a particle (e.g. powder) feeder 1018, a gas heater1014, and an array of spray nozzles 1024. The spray system 1000 of 10Ais configured in a parallel arrangement. The spray system 1010 of 10B isconfigured in an in-line arrangement. In some embodiments, a spraysystem may utilize a parallel arrangement or an in-line arrangement. Insome embodiments utilize only one each of said components. Otherembodiments utilize multiple of one or more said components such asmultiple arrays of nozzles connected to one system or multipleindividual systems of gas heaters and nozzle arrays sharing one centralgas supply.

Metal Particles

Illustrative embodiments utilize metal particles (e.g., powder) as aconsumable input. These particles are fed into the carrier gas, where itis carried to the substrate at a high speed. The particles then impactthe substrate (e.g., metal component), and form a metallurgical bond tothe surface of the substrate, thus forming the coating. The particlescan be fabricated through multiple methods including gas atomization,water atomization, and mechanical crushing.

In some embodiments, the applied particles are spherical particlesfabricated through gas atomization. In some embodiments, the appliedparticles are near-spherical particles fabricated through high pressurewater atomization. In some embodiments, the applied particles areirregular shaped particles fabricated through mechanical crushing.

In some embodiments, the particles have mean particle sizes in the rangeof 5-20 microns. In some embodiments, the applied coatings have a meanthickness in the range of 10-100 microns.

In some embodiments, the applied particles are a stainless steel such asa 300 series or 400 series steel to impart corrosion resistance to asteel component. In other embodiments, other corrosion resistantmaterials such as aluminum are used in the coating. In otherembodiments, a sacrificial metal such as zinc is used to providecathodic protection against corrosion.

In some embodiments, ceramic particles or other non-metal materials areincorporated into the metal coating to increase hardness or modify othersurface properties.

Carrier Gas

Illustrative embodiments utilize a high pressure carrier gas totransport and apply kinetic energy to the coating particles. In someembodiments the carrier gas is supplied by a pressurized gas tank thatis periodically filled or replaced. In some embodiments, the carrier gasis supplied by a gas compressor. In some embodiments, the pressure ofthe carrier gas may be between 500 pounds per square inch (e.g., psi)and 1000 psi, or the pressure may be between 600 psi and 900 psi, or thepressure may be between 700 psi and 800 psi, or the pressure may beabout 725 psi.

The carrier gas (e.g., gas) may be preheated in a gas heater integratedinto the cold sprayer. The carrier gas may also be heated prior to beingprovided into the cold sprayer. In some embodiments, the temperature ofthe gas may be between 700 C and 1300 C, or the temperature of the gasmay be between 850 C and 1150 C, or the temperature of the gas may bebetween 900 C and 1100 C, or the temperature of the gas may be about1000 C.

In some embodiments, a range of carrier gases may be used, including:nitrogen (N₂), helium (He), a mixture of nitrogen (N₂) and helium (He),air, and argon (Ar). In some embodiments, a reactive gas such as forminggas (e.g., 5% H₂ in N₂) may be used. Other embodiments will utilizeother carrier gases or mixtures of gases.

Nozzles

FIG. 11 schematically illustrates various embodiments of the presentdisclosure of a nozzle design in accordance with illustrativeembodiments. In illustrative embodiments, a convergent-divergent nozzledesign 1100 is utilized to accelerate the carrier gas and coatingparticles to supersonic velocities. A high pressure, high temperaturecarrier gas is mixed with the coating powder and fed into the nozzle1110. As the gas and particles enter the nozzle, the gas is at highpressure, high temperature, and at subsonic velocities 1110 This gas iscompressed through the converging section of the nozzle 1120, decreasingthe pressure and temperature of the gas, but at the same time reachingthe speed of sound at the throat of the nozzle 1125. The gas is thenexpanded through the diverging section of the nozzle 1130, where thepressure and temperature decrease further, and the gas velocityincreases above the speed of sound. The coating particles are carried bythe gas through both converging and diverging sections of the nozzle,and are accelerated through the force imparted by the high velocity ofthe gas. The carrier gas and powder exit the nozzle, forming a sprayplume. That spray plume is directed onto the substrate, where thecoating particles impact with high velocity and bond to the surface. Insome embodiments, the convergent-divergent nozzle design is utilized inmultiple such nozzles surrounding the component of interest to achieve afull coating.

Illustrative embodiments utilize an adaptation of the cold spraydeposition process as a high throughput, low cost method for applyingcorrosion resistant coatings in a manner that is integrated intoexisting manufacturing processes for steel and other metals. In someembodiments disclosed herein, multiple, stationary convergent-divergentspray nozzles oriented in a way to fully coat the manufactured steel asit passes through an automated manufacturing line. Furthermore, otherembodiments disclosed herein utilize multiple, movableconvergent-divergent spray nozzles that move in a way to fully coat themanufactured steel as it passes through an automated manufacturing line.

It should be noted that although steel components are discussed, variousembodiments apply to other metals, including aluminum, copper, and/ornickel based alloys. Furthermore, various embodiments apply to othergeometries, including pipes, tubes, beams, and girders. While preventionof corrosion is discussed, various embodiments modify other surfaceproperties including hardness, roughness, wear resistance, and visualappearance. Accordingly, discussion of steel bars and corrosion is forillustrative purposes and not intended to apply to all embodiments.

Some embodiments apply a corrosion resistant coating to existing steeland/or other metal products without significant adaptation of themanufacturing process. Such coatings impart corrosion resistance to theproduct, extending the shelf life and operational lifetime of theproduct and increasing the value of the product to its user.

Various embodiments may be incorporated into an existing metalmanufacturing process at any step that enables line-of-sight access tothe product. Some embodiments may apply a coating to a substrate acrossa full range of temperatures expected during the steel manufacturingprocess. The applied coating preferably does not significantly impactthe shape or structural properties of the underlying metal.

Nozzle Arrays

FIGS. 12A and 12B schematically illustrate various embodiments of thepresent disclosure of nozzle arrays in accordance with illustrativeembodiments. FIG. 12A shows an embodiment 1200 of a rectangular nozzlearray 1210 of cold spray nozzles 1220 that is oriented to coat theoutside of a rectangular steel component 1250 (e.g., substrate), such asa steel billet. A coating spray 1230 is deposited on the rectangularsteel component 1250 to form an applied coating 1240. The nozzles 1220in the array of nozzles 1210 are configured to cover the entire surfaceof the rectangular steel component 1250. That is, the array of spraynozzles 1210 circumscribes the rectangular steel component 1250 in arectilinear configuration to ensure there is an unobstructed line ofsight between each region of the surface of the rectangular steelcomponent 1250 and at least one of the nozzles 1220 in the array ofspray nozzles 1210.

FIG. 12B shows an embodiment 1255 of a circular nozzle array 1260 ofcold spray nozzles 1270 that is oriented to coat the outside of acircular steel component 1250 (e.g., substrate), such as rebar. Acoating spray 1280 is deposited on the circular steel component 1295 toform an applied coating 1290. The nozzles 1270 in the array of nozzles1260 are configured to cover the entire surface of the circular steelcomponent 1295. That is, the array of spray nozzles 1270 circumscribesthe circular steel component 1250 in a circular configuration to ensurethere is an unobstructed line of sight between each region of thesurface of the circular steel component 1295 and at least one of thenozzles 1260 in the array of spray nozzles 1270. In other embodiments, acomplex array of nozzles are used to coat complex shapes includinggirders and beams.

Nozzle Placement

In illustrative embodiments, the nozzles in a nozzle array are orientedto ensure full coverage of the coated product. In some embodiments,nozzles are oriented such that the spray tracks of each nozzle overlapwith neighboring spray tracks to ensure no gaps in coverage exist. Insome embodiments, some of the spray nozzles may be oriented at leastpartially in a longitudinal direction and at least partially in a radialdirection. This may enable the array of nozzles to cover portions of thecomponents that might not be covered with an array of spray nozzles thatare positioned only in a single plane. That is, at least some spraynozzles in an array of spray nozzles may produce a stream of particlesat least partially in a longitudinal direction and at least partially ina radial direction. One or more nozzles may also point off of a radialline that is projected through the component. The direction of theoff-radial line may be a tilt off of a plane formed by a collection ofradial lines.

FIG. 13 schematically illustrates various embodiments of the presentdisclosure of nozzle arrays in accordance with illustrative embodiments.FIG. 13 (left) provides an illustrative example of embodiments of anozzle array having a flat circumscribed array 1300 having the spraynozzles 1310 in a single plane. A substrate (e.g., metal component) 1320is coated with coating 1330.

FIG. 13 (right) provides an illustrative example of embodiments of anozzle array having a flat staggered array 1350 of coating nozzles 1360.In this embodiment, the array 1350 has the spray nozzles 1360 in aspiral array. A substrate (e.g., metal component) 1370 is coated withcoating 1330.

In some embodiments involving coating of products with raised or sunkenfeatures will rotate nozzles relative to the surface in order to achievefull coverage. In one such embodiment, coating of steel rebar requirescoating of ridges on the surface of the rebar.

FIG. 14 schematically illustrates various embodiments of the presentdisclosure of nozzle arrays in accordance with illustrative embodiments.FIG. 14 (left) illustrates an embodiment where nozzles 1310 areconfigured to spray 1330 particles to cover ridges 1320 on the surfaceof rebar. In FIG. 14 (left) a nozzle 1310 is located to one side of theridge and directed towards the ridge, to ensure that the coating 1340covers the sides of the ridges.

FIG. 14 (right) illustrates an embodiment 1350 where nozzles 1360 areconfigured to cover ridges 1320 on the other side surface of rebar(relative to FIG. 14 (left)). In FIG. 14 (right) a nozzle 1360 islocated to the other side of the ridge 1320 (relative to FIG. 14 (left))and directed towards the ridge and the surface, to ensure that thecoating 1380 covers the sides of the ridges, as well as the surface.

FIG. 15 schematically shows an illustrative embodiment where a laterspray nozzle is oriented to overlap with the coating track from aprevious nozzle. First spray nozzle 1510 sprays a first coating spraytrack 1520 on component 1505 that is traveling in direction 1515relative to the first spray nozzle 1510. Second spray nozzle 1530 spraysa second coating spray track 1540. A coating overlap 1550 is produced bythe overlapping spray nozzles 1510 and 1540.

FIG. 16 schematically shows an illustrative embodiment 1600 whereconsecutive arrays of spray nozzles 1604 are configured to reach adesired coating thickness. In such embodiments, each array contributes aportion of the coating thickness, and each subsequent array builds onthe thickness of the previous array. As schematically shown in FIG. 16 ,the first array 1610 sprays on first coating 1620 on steel substrate1608 (e.g., steel component). The second array 1630 sprays on secondcoating 1640, and the third array 1650 sprays on third coating 1660. Inthis way, applied coating 1606 may attain the required thickness.

In some embodiments, arrays of spray nozzles are identical andinterchangeable. In other embodiments, the arrays are located anddirected differently to minimize local variations in the coatingthickness. In some embodiments, a number of arrays are assembled inseries in order to achieve the required coating thickness. In someembodiments, nozzles are oriented to cover the full circumference of thebar, including coverage of any ridges or other geometric features.

Integration of the Coating System

In illustrative embodiments, a spray coating system may be integratedinto conventional metal manufacturing processes at many potential stepswhere line of sight access is possible. In some embodiments, coatingwill be performed near the end of the manufacturing process, once theproduct has been processed into its final shape. In other embodiments,the coating step could occur prior to or in between rolling steps, withthe coating product bond maintained through the rolling process. Inother embodiments, the coating could occur prior to heating. In otherembodiments, coating could occur after cutting of the product.

FIG. 17A schematically illustrates various embodiments of integration ofspray systems into a manufacturing line in accordance with illustrativeembodiments. FIG. 17A schematically shows a coating system installedinto a steel bar manufacturing process 1700. In this embodiment, thecoating system is located after the majority of the processing steps.Briefly, the manufacturing steps of heating 1710, rolling 1720, andquenching 1730 of the bar occur before the coating is applied 1740. Incoating 1740, the coating system then applies a stainless steel coatingin this embodiment through several stages of circular arrays. After thecoating 1740, the coated steel bar is cut to length, allowed to cool,and bundled for distribution.

FIG. 17B schematically shows a coating system installed into a steel barmanufacturing process 1700. In this embodiment, the coating system islocated before the majority of the processing steps. Briefly, thecoating 1770 is performed on a steel billet prior to the steelmanufacturing steps. In this embodiment, the coating system applies astainless steel coating through several stages of circular arrays.Following the coating of the billet with the stainless steel coating,the coated billet goes through the processing steps of heating 1775,rolling 1780, and quenching 1785 of the bar. After quenching, the coatedsteel bar is cut to length, allowed to cool, and bundled fordistribution.

Other embodiments include integration into manufacturing processes withother processing steps including casting, extruding, and drawing.Coating systems can be integrated prior to or after such processingsteps.

Some illustrative embodiments integrate coatings into stages where theproduct has a minimal amount of oxide scale to limit contamination ofthe applied coating. Examples of such locations include immediatelyafter high pressure water quenching and immediately after a rolling,drawing, and other deformation processes. In other embodiments, anadditional processing step is used to strip the product of any oxidescale prior to coating.

Sizing and Orientation of the Coating System

The size of illustrative embodiments, including the number of nozzlesand coating throughput, varies and is dependent on the details of theexisting manufacturing process.

In some embodiments, the minimum coating thickness is determined by therequirements of the application. For instance, some applications willhave a known maximum corrosion rate, requiring a particular coatingthickness to ensure the component is protected for its full servicelife. Other applications will have requirements for the mechanicaldurability of the coating, including wear or scratch resistance thatrequire a coating of a specific thickness to be met. In otherembodiments, the minimum coating thickness will be determined by theneed to reach full coverage of the component. In said embodiments, thecoating thickness would be at least two to four times the mean diameterof the coating powder in order to enable full coverage.

A required coating throughput of embodiments can be determined by thedesired coating thickness, the throughput of the existing manufacturingprocess, and the surface area of the product to be coated. The requiredthroughput can then determine a minimum nozzle number, based on themaximum coating deposition rate of each nozzle. The coating system canthen be sized to ensure it both meets the minimum nozzle number and canfully cover the surface of the coated component given the coating areaof each nozzle.

Cold Spraying Large Steel Billet

A large billet of steel (approximately 7 in×7 in×25 ft) can be coated,rolled, and processed so that the delivered rebar already has a coatingon it. In some embodiments, a process similar to that described abovefor FIG. 17B can be used to coat a large billet prior to the billetbeing processed into steel bars. By applying the stainless steel coatingto a large billet prior to the heating, rolling, and quenching steps, asdescribed for FIG. 17B, the costs of manufacturing of pre-coated rebarmay be lower, and the throughput may be higher than coating the rebarafter the heating, rolling, and quenching steps, as illustrated in FIG.17A.

The surprising results of preparing a coating of a corrosion resistantferrite stainless steel matrix on a steel component, indicates thatusing an array of spray nozzles in a cold spray process can be used tomake corrosion resistant steel components.

New Steel Alloy Composition (Fe—Cr—Si—Al—Mo Alloys) (3)

Atmospheric and aqueous corrosion of steel is a major challenge to theengineering of effective and durable structures. Modifying thecomposition of steel to limit corrosion can reduce maintenance cost andenhance performance, as is the case with stainless steels. Corrosionresistance in steels is typically achieved by adding 14-20 wt. %chromium in order to form a surface layer of Cr₂O₃ (chromia) whichprotects the steel from corrosion. But, the chromia layer is susceptibleto attack in a wide variety of settings including different corrosiveenvironments, acidity (pH), and temperature. In particular, attack byhalides including chloride ions (e.g., Cl⁻) and fluoride ions (e.g., F⁻)can remove the protective oxide layers and cause aggressive pitting-typecorrosion.

In order to allow general corrosion resistance and halide resistance ina ferritic steel, illustrative embodiments use high concentrations ofmolybdenum (e.g., Mo) alongside passivating oxide formers such aschromium, aluminum, and silicon. Ferritic steels typically have a bodycentered cubic (e.g., BCC) structure. These high concentrations ofmolybdenum typically rapidly re-passivate regions where the oxide layeris damaged by halide attack. This results in a corrosion resistantstainless steel that has far greater resistance to halide attack thanexisting ferritic steels. Furthermore, 316 Stainless steel includes 2 wt% Mo to provide corrosion resistance in chloride environments.

In some embodiments, silicon and aluminum are incorporated as secondaryand tertiary oxide formers, enabling further corrosion resistance in abroader range of environments. The presence of austenite stabilizingelements such as nickel and manganese are limited to preserve theferritic phase. Alloys such as Fe—Cr—Si and Fe—Cr—Al utilize the conceptof a secondary protective oxide layer.

In illustrative embodiments, ferritic stainless steels can be utilizedas a corrosion resistant coating to other compositions of matterincluding carbon steels. This outer coating could be applied by avariety of application methods including cold spray, weld overlay, andco-extrusion.

Application of Fe—Cr—Mo—Al—Si Alloys for Corrosion Resistance

FIG. 18 schematically illustrates embodiments of Fe—Cr—Mo—Al—Sistainless steel alloys as a corrosion resistant coating on steelcomponents. FIG. 18 shows a schematic embodiment of a multilayer oxidelayer that illustrates how the metal atoms in an alloy diffuse toindividual oxide layers on the surface of the alloy. The bottom materialin FIG. 18 is a steel component 1810. A Fe—Cr—Mo—Al—Si alloy coating hasbeen deposited on the steel component, and the aluminum, silicon, andchromium elements have diffused to form a multilayer coating of metaloxide layers 1870. The aluminum oxide layer 1820 is closest to the steelcomponent 1810. The next layer up from the aluminum oxide layer is asilicon dioxide layer 1830. The chromium oxide layer 1840 is on thesurface of the multilayer oxide coating. 1870.

A pit (e.g., crevice) has been formed by chloride ion (Cl⁻) corrosionthrough the multilayer oxide coating and extending into the steelcomponent. While the oxide layers in the multilayer oxide coating areeffective at preventing oxidative corrosion from oxygen, they are lesseffective at preventing Cl⁻ pitting corrosion. However, molybdenum atomsin the stainless steel diffuse to the pitting surfaces caused by the Cl⁻pitting corrosion, and form a MoO₃ passivating native oxide layer 1860on the corroded surface. The MoO₃ passivating layer prevents, and/orresists further corrosion by the chloride ions.

FIG. 19 shows an Ellingham diagram presenting the relative driving forcefor the oxide formation among the constituent elements in the presentinvention. An Ellingham diagram is a graph showing the temperaturedependence of the stability of compounds. The Ellingham diagram plotsthe Gibbs free energy change (ΔG) for each oxidation reaction as afunction of temperature. The diagram has particular utility inidentifying which metal oxide layers in a multilayer oxide coating arelikely to form the most stable oxides (having the largest absolute valueof ΔG), and thus allow the determination of the stacking order of theoxide layers. This is because the more stable oxide layers will tend toform closer to the steel component.

As shown in FIG. 19 , Al₂O₃ has the largest absolute value of ΔG, and itis the layer closest to the steel layer. SiO₂ has the next largestabsolute value of ΔG, and it is the layer on top of the Al₂O₃ layer.Cr₂O₃ has the smallest absolute value of ΔG, and it is the layer on topof the multilayer oxide coating.

The selection of chromium, molybdenum, aluminum, and silicon as theprimary, secondary, and tertiary corrosion resistant stainless steelalloy metals is based on optimizing the structure and chemicalresistance of the alloy. The structure is optimized to form a bodycentered cubic (e.g., BCC) ferrite matrix, and the chemical resistanceis optimized to resist corrosion of steel by both oxygen and chloride.

FIG. 20 shows equilibrium phase compositions for ranges of Cr and Mocontents in stainless steel alloys of iron, chromium, and molybdenum.The elemental ranges in an exemplary composition of matter may be chosenusing following considerations. Iron (Fe) is selected as the primaryelement for the alloy due to its abundance, affordability, strength, andductility. Chromium (Cr) is selected as the main alloying element inorder to achieve overall corrosion resistance for the alloy, which isachieved by having a minimum 16 wt % Cr in order to form a stable,protective Cr₂O₃ layer while having no more than 20 wt % Cr in order tolimit or prevent the formation of deleterious metallic phases such assigma phase and laves phase. Molybdenum (Mo) is included in a minimum of3 wt % in order to achieve additional corrosion resistance inconcentrated halide media, especially high chloride aqueousenvironments, and is limited to no more than 4-8 wt %, in order to limitthe formation of chi phase, sigma phase, and laves phase intermetallics.Aluminum (Al) can be included in order to form an additional Al₂O₃protective oxide layer, with a limit of no more than 4 wt % in order toavoid the formation of brittle Fe₃Al precipitates. Silicon (Si) can alsobe included in order to form an additional SiO₂ protective oxide layer,with a limit of no more than 2 wt % in order to avoid the formation ofbrittle Cr₃Si precipitates. Manganese (Mn) is included in a small amount(0.1-0.5 wt %) in order to consume sulfur impurity in the molten ironvia the formation of small MnS precipitates. Carbon (C) and nitrogen (N)are limited to 0.1 wt % to limit the formation of carbide and nitridephases in the material. Sulfur (S) is limited to 0.05 wt % in order toavoid the severe embrittlement that sulfur can cause to steels.

Range of Potential Compositions

Various embodiments of the invention have the following elements withinthe specified ranges to achieve the microstructure and utility describedbelow.

In accordance with an embodiment of the invention, a corrosion resistantstainless steel alloy composition having a BCC ferrite matrix includes:

-   -   12-25 weight percent chromium (Cr);    -   2-10 weight percent molybdenum (Mo); and at least one or more        of:    -   0-10 weight percent aluminum (Al);    -   0-5 weight percent silicon (Si);    -   0-5 weight percent nickel (Ni);    -   0-1.0 weight percent manganese (Mn);    -   0.0-0.1 weight percent carbon (C);    -   0.0-0.1 weight percent nitrogen (N); and    -   0.0-0.05 weight percent sulfur (S); and    -   the balance of iron (Fe).

In accordance with an embodiment of the invention, a corrosion resistantstainless steel alloy composition having a BCC ferrite matrix includes:

-   -   16-20 weight percent chromium (Cr);    -   3-6 weight percent molybdenum (Mo); and at least one or more of:    -   0-4 weight percent aluminum (Al);    -   0-2 weight percent silicon (Si);    -   0-0.1 weight percent nickel (Ni);    -   0.1-0.5 weight percent manganese (Mn);    -   0.0-0.1 weight percent carbon (C);    -   0.0-0.1 weight percent nitrogen (N):    -   0.0-0.05 weight percent sulfur (S); and    -   the balance of iron (Fe).

In accordance with an embodiment of the invention, a corrosion resistantstainless steel alloy composition having a BCC ferrite matrix includes:

-   -   18 weight percent chromium (Cr);    -   6 weight percent molybdenum (Mo);    -   4 weight percent aluminum (Al);    -   2 weight percent silicon (Si); and    -   the balance of iron (Fe).

In accordance with an embodiment of the invention, a corrosion resistantstainless steel alloy composition having a BCC ferrite matrix includes:

-   -   18 weight percent chromium (Cr);    -   3 weight percent molybdenum (Mo);    -   4 weight percent aluminum (Al);    -   2 weight percent silicon (Si); and    -   the balance of iron (Fe).

In accordance with an embodiment of the invention, a corrosion resistantstainless steel alloy composition having a BCC ferrite matrix includes:

-   -   18 weight percent chromium (Cr);    -   8 weight percent molybdenum (Mo);    -   5 weight percent aluminum (Al); and    -   the balance of iron (Fe).

In accordance with an embodiment of the invention, a corrosion resistantstainless steel alloy composition having a BCC ferrite matrix includes:

-   -   18 weight percent chromium (Cr);    -   8 weight percent molybdenum (Mo);    -   2 weight percent silicon (Si); and    -   the balance of iron (Fe).

In accordance with an embodiment of the invention, a corrosion resistantstainless steel alloy composition having a BCC ferrite matrix includes:

-   -   18 weight percent chromium (Cr);    -   4 weight percent molybdenum (Mo); and    -   the balance of iron (Fe).

General Method of Fabrication

Formation Steps

To synthesize the alloy, one skilled in the art can employ the followingsteps. It should be noted that this method is substantially simplifiedfrom a longer process that normally would be used to synthesize thealloy. Accordingly, the process of synthesizing the alloy is expected tohave many steps that those skilled in the art likely would use. Inaddition, some of the steps may be performed in a different order thanthat shown, or at the same time.

Those skilled in the art therefore can modify the process asappropriate. Moreover, as noted above and below, the materials andstructures noted are but one of a wide variety of different materialsand structures that may be used. Those skilled in the art can select theappropriate materials and structures depending upon the application andother constraints. Accordingly, discussion of specific materials andstructures is not intended to limit all embodiments.

One process of various embodiments has the following steps.

-   -   Provide a mixture of the raw materials in the appropriate weight        fractions, as described above for a corrosion resistant        stainless steel alloy composition having a BCC ferrite matrix.    -   Provide a furnace for melting the metal mixture.    -   Heat the metal mixture in the furnace to a temperature between        about 1600 C and about 2000 C to form a liquid metal mixture        melt.    -   Cool the liquid metal mixture melt to an intermediate        temperature between about 1000 C and about 1300 C over a first        duration of time to initiate a solidification process.    -   Quench the liquid metal mixture melt to a temperature between        about 400 C and about 600 C over a second duration of time,        because the quenching limits the formation of carbide        precipitates.    -   Temper the metal mixture at the temperature between about 450 C        and about 600 C for a duration of time of between about 10        minutes and about 60 minutes.    -   Cool the metal mixture in the absence of active heating, wherein        the metal mixture comprises ferritic stainless steel alloy.

The first duration of time may be between 5 minutes and 100 minutes. Thesecond duration of time may be between 0.5 seconds and 10 seconds.

EXAMPLES

The following examples are intended to further illustrate the disclosureand its preferred embodiments.

Example 1: Cost Effective Option

Grade 304 SS Coating, 20-40 Micron Coating Thickness, Cold SprayDeposition

Example 1 represents an embodiment of the invention which is mostoriented towards reduction in costs. This example employs 304 SS as thecorrosion-resistant coating layer due to its good general corrosionresistance as well as the affordability of this coating material incomparison to other suitable corrosion-resistant coating materials.

Cold spray deposition is employed as the method of applying the coatingto the carbon steel bar due to its relatively low cost and highthroughput. A coating thickness of 20-40 microns is selected becausethis thickness range allows for confidence that the corrosion resistantcoating is fully covering the surface of the material, while alsolimiting the use of the coating material in order to reduce the overallcost of the coated bar. The cold spray deposition should be performednear the end of the production process on the finished or near-finishedbar.

Example 2

Grade 316 SS Coating, 25-50 Micron Coating Thickness, Cold SprayDeposition

Example 2 represents an embodiment of the invention in which substantialchloride corrosion resistance is achieved in addition to generaloxidation resistance, while reducing the cost but using a relativelythin coating layer. This example uses 316 SS as the corrosion-resistantcoating layer which provides the protection of a conventional stainlesssteel with substantial additional corrosion resistance to chlorideattack due to the increased molybdenum content.

Cold spray deposition is chosen in this example due to the ability toinexpensively produce coating of the appropriate thickness with a highthroughput. A coating thickness of 25-50 micron is selected in order toprovide robust corrosion resistance while limiting cost increase. Thecold spray deposition should be performed near the end of the productionprocess on the finished or near-finished bar.

Example 3

Grade 316 SS Coating, 40-80 Micron Coating Thickness, Weld Overlay

Example 3 represents an embodiment of the invention with substantialchloride corrosion resistance while also being more robust againstbending, scratching, or other processes which may damage the surfacecoating. Again, with grade 316 SS used as the corrosion-resistantcoating layer, this example will provide significant resistance tocorrosion in chloride environments.

In this example, weld overlay is used to deposit the coating materialonto the bar. The coating thickness of 40-80 micron in this exampleprovides a more robust, mechanically sound coating which can morereadily withstand deformation of the bar while maintaining corrosionprotection. As such, the weld overlay process should occur towards thebeginning of the bar production process. For example, the weld overlayprocess may be applied to coating a large billet which is rolled down tothe finished bar dimension.

Example 4

Fe-18Cr-6Mo-4Al-2Si SS, 25-50 Micron Coating Thickness, Cold SprayDeposition

Example 4 is an embodiment which utilizes a highly corrosion-resistantcoating material, Fe-18Cr-6Mo-4Al-2Si. This alloy provides exceptionaloverall corrosion resistance, employing multiple elements which formprotective, passive oxide films in addition to chloride attackresistance provided by Mo. Although this coating material typically ismore expensive, the coating thickness of 25-50 microns which is selectedfor this example allows for a balance between performance and cost.

Cold spray deposition is chosen in this example due to the ability toproduce coating of the appropriate thickness with a high throughput. Thecold spray deposition should be performed near the end of the productionprocess on the finished or near-finished bar.

Example 5

Fe-18Cr-6Mo-4Al-2Si SS, 40-80 Micron Coating Thickness, Weld OverlayDeposition

Example 5 provides robust bar protection and performance, utilizing aFe-18Cr-6Mo-4Al-2Si alloy. A relatively thick coating of 40-80 micronallows the coating to withstand significant deformation, scratching, andother damage which could degrade the performance of the coating.

For this example, weld overlay is used in order to deposit theprotective coating material onto the bar. As such, the weld overlayprocess should occur towards the beginning of the bar productionprocess, with the weld overlay applied to a large billet which is rolleddown to the finished bar dimension.

Example 6

Maximum Corrosion-Resistance Composition; Fe-18Cr-6Mo-4Al-2Si—0.1C

This example demonstrates optimization of the composition for the bestcorrosion resistance possible by maximizing the content of theprotective oxide formers: Cr, Al, and Si along with Mo for additionalresistance to halide corrosion attack.

FIG. 21 shows an equilibrium phase fraction plot for an exampleFe—Cr—Mo—Al—Si composition demonstrates the secondary phases that shouldbe limited or avoided completely. Time-temperature-transformation curvefor an example Fe—Cr—Mo—Al—Si composition demonstrates how thetemperature profile can be controlled in order to limit/avoid theformation of secondary phases.

FIG. 22 illustrates time-temperature-transformation curve for an exampleFe—Cr—Mo—Al—Si composition demonstrates how the temperature profile canbe controlled in order to limit/avoid the formation of secondary phases.

Due to the large Mo content in this composition, carbide formation isespecially rapid for the M6C phase, so rapid quenching down to 900K(˜625 C) is critical to limit the overall volume fraction of carbideprecipitates, followed by more gradual cooling down to room temperatureafter quenching. The benefit of this large Mo content is that resistanceto chloride corrosion attack should be excellent.

Example 7

Balanced Multi-Oxide Composition; Fe-18Cr-3Mo-4Al-2Si

This example demonstrates a composition with reduced Mo concentration inorder to reduce the cost of the material and reduce the likelihood offorming secondary phases, especially carbide precipitates and Lavesphase precipitates. Overall this composition should still have excellentoverall corrosion resistance, but less resistance to chloride attackcompared with example #1. But, the cost is reduced by Mo contentreduction, and the thermal processing requirements are somewhatalleviated as well.

FIG. 23 shows a phase composition map for Fe-18Cr-3Mo-4Al-2Si shows thepresence of deleterious secondary phases at low temperatures, especiallyCr₃Si, which are limited in the final microstructure through thermalprocess control.

FIG. 24 shows a time-temperature-transformation diagram forFe-18Cr-3Mo-4Al-2Si, demonstrating the kinetics of formation ofsecondary phases in this example

Given the kinetic information demonstrated above in FIG. 7 , thereshould be a rapid quench of the material, lowering the temperature from1200K to 900K as rapidly as possible to minimize the formation ofcarbide precipitates. Following the quench, the material is moregradually cooled from 900K to 700K, followed by air cooling from 700K toambient conditions.

Example 8

Double Oxide—Chromium and Aluminum; Fe-18Cr-8Mo-5Al

This example demonstrates a composition protected by chromia and aluminalayers without the use of silicon. Molybdenum is included forhalide/pitting protection.

Example 8 achieves corrosion resistance through the inclusion of Cr,along with additional significant halide corrosion resistance due to thelarge Mo content. The presence of Al allows for the formation of asecondary protective alumina passive layer which further enhances thecorrosion resistance of this composition. The exclusion of Si from thiscomposition is expected to somewhat lower the oxidation resistance ofthis composition relative to other example compositions, but theformation of Cr₃Si is no longer thermodynamically favored as well, whichis beneficial to the microstructure of the material.

FIG. 25 shows an equilibrium phase fraction plot for an exampleFe—Cr—Mo—Al composition demonstrates the secondary phases that should belimited or avoided completely.

FIG. 26 shows a time-temperature-transformation diagram forFe-18Cr-8Mo-5Al, demonstrating the kinetics of formation of secondaryphases in this example

For example 8, the primary deleterious phase of concern is Laves phase,which begins to be thermodynamically favorable below 1000 K and becomeskinetically favorable (begins to significantly nucleate and grow) around900 K. During the synthesis of this material, a rapid quench should beperformed in order to rapidly cool the material from 950 K to 750 K inorder to limit the formation of Laves phase precipitates which tend toembrittle the material. After quenching to 750 K, the material can begradually cooled to ambient conditions.

Example 9

Double Oxide; Fe-18Cr-8Mo-2Si

Example 9 achieves corrosion resistance through the inclusion of Cr,along with additional significant halide corrosion resistance due to thelarge Mo content. The presence of Si allows for the formation of asecondary protective silica (SiO₂) passive layer which further enhancesthe corrosion resistance of this composition. The exclusion of Al fromthis composition is expected to somewhat lower the oxidation resistanceof this composition relative to other example compositions, but shouldreduce the potential for nitride and carbide precipitation. Especiallynotable phases that can potentially form are the chi and sigma phases,which could embrittle the material if the volume fraction of theseprecipitates becomes too large. In order to limit the volume fraction ofthese phases, the material should be quenched in order to rapidly coolthe material from 1200 K to 900 K, followed by gradual cooling toambient temperature.

FIG. 27 illustrates an equilibrium phase fraction plot for anFe—Cr—Mo—Si alloy demonstrates the secondary phases that should belimited or avoided completely.

FIG. 28 shows a time-temperature-transformation diagram forFe-18Cr-8Mo-2Si demonstrating the kinetics of formation of secondaryphases in this example.

Example 10

Single Oxide—Fe-18Cr-4Mo

Example 10 is a simple example, where the only alloying additions to Feare Cr and Mo. Good overall corrosion resistance is achieved through thelarge Cr concentration, while a significant additional resistance tochloride corrosion attack is achieved through the addition of 4 wt % Mo.The Cr and Mo concentrations are low enough that sigma phase formationis not thermodynamically favored, but chi phase can form during thematerial synthesis process, and therefore certain processing stepsshould be taken to avoid formation of chi phase and resultingembrittlement of the material.

FIG. 29 illustrates an equilibrium phase fraction plot for an Fe—Cr—Moalloy demonstrates the secondary phases that should be limited oravoided completely.

FIG. 30 shows a time-temperature-transformation diagram for Fe-18Cr-4Modemonstrating the kinetics of formation of secondary phases in thisexample

In order to minimize the formation of chi phase in the synthesis ofExample 10, quenching must be performed in order to rapidly lower thetemperature of the material from 1100 K to 850 K, followed by holdingthe material at 800 K for 10 minutes to briefly anneal out any defectsand possible chi phase which may have formed during the solidificationprocess. Following this brief annealing step, the material can be aircooled from 800 K to ambient conditions.

Example 11

Further Examples of Compositions with Various Amounts of AlloyingElements

Embodiments include compositions with various amounts of alloyingelements, including those with other elements not listed here. Furtherexamples of compositions within the scope of the present inventioninclude:

Fe-12Cr-2Mo; Fe-12Cr-5Mo-1Si—0.4Mn-0.3C; Fe-22Cr-6Mo-0.2Mn;Fe-22Cr-3Al-2Mo-1Si—0.2Mn-0.2C; Fe-20Cr-8Mo-4Al-2Si; Fe-22Cr-3Al-0.1C;Fe-18Cr-2Mo-1Si—0.3Mn; Fe-12-Cr-2Mo-1W; Fe-12-Cr-2Mo-0.15V; andFe-16Cr-4Ni-2Mo.

The embodiments of the invention described above are intended to bemerely exemplary; numerous variations and modifications will be apparentto those skilled in the art. Such variations and modifications areintended to be within the scope of the present invention as defined byany of the appended claims.

1. A coating deposition system for applying a coating of stainless steelto a surface of a steel component, the system comprising: a gas inputfor fluidly coupling with a high pressure gas supply configured toprovide a gas at a high pressure to one or more flow paths; a heated gasflow path in thermal communication with a gas heater, the heated gasflow path in fluidic communication with the high pressure gas supply,the gas heater configured to heat the high pressure flowing gas in theheated gas flow path; a stainless steel particle feeder flow path inparticulate communication with a feeder input for receiving a source ofstainless steel particles, the stainless steel particle feeder flow pathin fluidic communication with the high pressure gas supply, thestainless steel particle feeder configured to supply the stainless steelparticles to the high pressure flowing gas in the stainless steelparticle feeder flow path; and an array of spray nozzles in fluidiccommunication with the heated gas flow path and the stainless steelparticle feeder flow path, the array of spray nozzles in particulatecommunication with the stainless steel particle feeder flow path,wherein: the array of spray nozzles is configured to circumscribe thesteel component so that there is an unobstructed line of sight betweeneach region of the surface of the steel component and at least one ofthe nozzles in the array of nozzles; the array of spray nozzles isconfigured to accelerate the stainless steel particles by a forceimparted by a high velocity of the heated gas exiting each of thenozzles in the array of nozzles in a plume of heated gas and stainlesssteel particles; the array of spray nozzles further configured so thatthe stainless steel particles impact the surface of the component at thehigh velocity and metallurgically bond to the surface of the componentto form the stainless steel coating, the stainless steel coating beingferritic, austenitic, or duplex; and at least one of the spray nozzlesproducing a stream of stainless steel particles at least partially in alongitudinal direction and at least partly in a radial direction.
 2. Thesystem of claim 1, wherein the array of spray nozzles is in particulatecommunication with the stainless steel particle feeder flow path andconfigured so that for each nozzle in the array of nozzles: the heatedgas and particles enter the nozzle; the heated gas is compressed througha converging section of the nozzle; the heated gas is then expandedthrough a diverging section of the nozzle, and after passing through theconverging and diverging sections of the nozzle, the heated gas andstainless steel particles exit the nozzle in the plume and impact thesurface of the steel component at supersonic velocities.
 3. The systemof claim 1, further comprising: a conveyor configured for thetransportation of the component through the plume of hot gas andstainless steel particles.
 4. The system of claim 1, wherein the gascomprises at least one of nitrogen (N₂), helium (He), air, argon (Ar),xenon (Xe), or forming gas (5% H₂ in N₂).
 5. The system of claim 1,wherein: the component has a rectilinear cross section; and the array ofspray nozzles is configured to circumscribe the rectilinear crosssection with heads of each of the nozzles in the array of spray nozzlesin a rectilinear arrangement.
 6. The system of claim 1, wherein; thehigh pressure is between about 700 psi and about 800 psi; thetemperature of the heated gas is between about 900 C and about 1100 C;and the high velocity is supersonic velocity.
 7. The system of claim 1,wherein the stainless steel particles have a mean particle size ofbetween 5 microns and 20 microns.
 8. The system of claim 1, wherein thestainless steel coating has a mean thickness of between 10 microns and100 microns.
 9. The system of claim 1, wherein: the stainless steelparticles comprise: 16-20 weight percent chromium (Cr); 3-6 weightpercent molybdenum (Mo); and at least one or more of: 0-4 weight percentaluminum (Al); 0-2 weight percent silicon (Si); 0-0.1 weight percentnickel (Ni); 0.1-0.5 weight percent manganese (Mn); 0.0-0.1 weightpercent carbon (C); 0.0-0.1 weight percent nitrogen (N): or 0.0-0.05weight percent sulfur (S); and the balance of iron (Fe).
 10. The systemof claim 9, wherein the stainless steel coating has a BCC ferritematrix.
 11. A method of applying a stainless steel coating to a steelcomponent, comprising: providing a carrier gas at a high pressure in afirst gas flow path to a gas heater to heat the carrier gas to a hightemperature along the first gas flow path; providing the carrier gas ata high pressure in a second gas flow path to a particle feeder ofstainless steel particles that are carried by the carrier gas along thesecond gas flow path; mixing the heated carrier gas in the first flowpath with the carried stainless steel particles in second flow path atan array of spray nozzles in fluidic communication with the first andsecond flow paths; and ejecting a plume of gas and stainless steelparticles from the array of nozzles to coat an outer surface of thesteel component with a coating of ferritic stainless steel as it istransported through the plume, wherein the ejected stainless steelparticles impact the surface of the steel component at a high velocityand form a metallurgical bond to the surface of the steel component toform an outer coating comprising stainless steel.
 12. The method ofclaim 11, wherein each of the nozzles in the array of nozzles:compresses the mixed heated carrier gas and particles through aconverging section of each nozzle; expands the mixed heated carrier gasand particles through a diverging section of each nozzle; andaccelerates the mixed heated carrier gas and particles to supersonicvelocities.
 13. The method of claim 11, wherein: the stainless steelparticles comprise at least one of 316 stainless steel, 2205 stainlesssteel, or 304 stainless steel.
 14. The method of claim 11, wherein thestainless steel outer coating comprises a ferritic stainless steel. 15.The method of claim 14, wherein: the steel component is a steel billet;the steel billet has a rectilinear cross section; the array of spraynozzles circumscribe the steel billet in a rectilinear configuration toensure there is an unobstructed line of sight between each region of thesurface of the billet and at least one of nozzles in the array of spraynozzles; the stainless steel coating covers the entire external surfaceof the steel billet; and the thickness of the stainless steel coating isbetween 150 microns and 2000 microns.
 16. The method of claim 15,wherein the method further comprises: heating the stainless steelcoating on the steel billet at 1200 C for a duration between about 3hours and about 9 hours; and hot rolling the stainless steel coating onthe steel billet to form a rebar component having the stainless steelcoating.
 17. The method of claim 11, wherein the stainless steel coatingcomprises at least one of 316 stainless steel, 2205 stainless steel, or304 stainless steel.
 18. The method of claim 11, wherein the stainlesssteel particles comprise at least one of: spherical particles fabricatedthrough gas atomization; near-spherical particles fabricated throughhigh pressure water atomization; or irregular shaped particlesfabricated through mechanical crushing.
 19. The method of claim 11,wherein the stainless steel has a ceramic material alloyed with thestainless steel to improve the bonding of the stainless steel coating tothe steel component.
 20. The method of claim 19, wherein the ceramicmaterial comprises at least one of a metal carbide or a metal oxide. 21.The method of claim 11, further comprising: heat treating the stainlesssteel coating on the steel component.
 22. The method of claim 21,wherein the heat treating the stainless steel coating on the steelcomponent comprises a laser heat treatment.
 23. The method of claim 20,wherein the heat treating the stainless steel coating on the steelcomponent comprises: heating the stainless steel coating on the steelcomponent to approximately 1100 C for 1 hour; quenching the stainlesssteel coating on the steel component to room temperature; and temperingthe stainless steel coating on the steel component at approximately 600C for 1 hour.